Q: Why do some humbucking pickups have covers, while others are open (that is, coverless)? Is it just for looks or is there a good reason for having a cover?

A: It depends upon whom you ask. In the 1960s, players with Les Pauls or other humbucker-equipped guitar (Clapton, Beck, Page, etc.) felt that removing the covers gave them a hotter, fatter tone. The earliest humbucking pickups, which were designed by Seth Lover at Gibson, first appeared on the 1957 Les Paul models. While the nickel- or gold-covered pickups were aesthetically pleasing, odds are good that if Lover felt they compromised the overall sound quality, he probably wouldn't have specified them. Another factor that may have influenced him to use covers was for the protection of the fragile wire wrapping on the magnets. Not many places had air conditioning in 1957, so he probably felt the covers would prevent sweat from getting inside the pickups and corroding the magnets or the wire wrapping. This is one of those discussions that will never end, as humbuckers have been around for more than half a century, and no one to date has shown definitive evidence for or against the use of covers, so it remains a personal choice. Most modern humbuckers are wax potted, making the removal of covers very difficult and quite risky, particularly if your guitar is still under warranty.